Virtually all silver halide grains utilized in photography exhibit face centered cubic ("rock salt") crystal lattice structures. Face centered cubic lattice structures are those lattice structures that have an internal ion arrangement--a crystal lattice--akin to the arrangement of ions in standard table salt (NaCl). Each lattice structure, barring imperfections or impurities which could distort the ionic arrangement, has similar ion types (anion or cation) occupying the corners and center of each face of a cube.
Although presently utilized silver halide grains have cubic lattices, such lattices, as noted, define an internal structure. The external appearance of grains defined by cubic lattices may be cubic, but this is not required. Silver halide grains containing cubic lattices may also take any one of a number of other morphologies.
Known morphologies for silver halide grains are as described in, for example, Maskasky, "The Seven Different Kinds of Crystal Forms of Photographic Silver Halides" Journal of Imaging Science, Vol. 30, 1986, pp. 247-255. It should be noted that known forms include the cube, octahedron, rhombic dodecahedron trisoctahedron, icositetrahedron, tetrahexahedron, and hexoctahedron.
In addition to describing silver halide grains according to their morphology, it is common practice in the art to describe grains by reference to their crystalline faces. Typically, Miller indices are utilized to define each face which bounds a silver halide grain. Miller indices, calculations thereof, and their manner of application are described in Crystals Perfect and Imperfect by A. Bennet, D. Hamilton, A. Maradudin, R. Miller and J. Murphy: Walker and Company, N.Y., 1965.
For cubic grains, the six crystal faces are usually referred to as {100} crystal faces, such reference being based upon the appropriate Miller indices. While the {100} crystal face designation is most commonly employed in connection with cubic silver halide grains, these same crystal faces are sometimes referred to as {200} crystal faces, the difference in designation resulting from a difference in the definition of the basic unit of the crystal structure.
Although {100} crystal faces represent the faces of a cubic grain, such faces may also be found in more irregularly shaped grains. An example of grains having a different morphology and yet also having {100} crystalline faces are the {100} silver chloride tabular grains of U.S. Pat. Nos. 5,264,337 and 5,275,930, and published European Patent Application 0 534 395. The grains of these references are tabular rather than cubic, and their primary faces have {100} Miller indices.
In addition to cubic grains, known silver halide grains also include octahedral grains. Octahedral grains have been determined to have {111} crystal faces. Like {100} crystal faces which are not limited merely to cubic grains, {111} crystal faces can be found in a multitude of other types of grains. An example of an irregular shaped grain having {111} major crystal faces can be found in Maskasky, "An Enhanced Understanding of Silver Halide Tabular Grain Growth", Vol. 31, 1987 pp. 15-26, which discusses trapezoid shaped grains having {111} major faces.
U.S. Pat. No. 4,643,966, discloses tabular grains having a ruffled surface. The ruffling of these grains is the result of protrusions emanating from the tabular surface of a {111} base plane. The protrusions may be small three sided "pyramids", each side of the pyramid having other than a {111} crystal face.
A third type of morphology of silver halide grains, although one that is much less common than the cubic or octahedral morphologies, is the rhombic dodecahedron. The rhombic dodecahedral grain is bounded by twelve identical faces. These faces are generally referred to as {110} crystal faces. As with {100} and {111} crystal faces, {110} crystal faces may be found in irregularly shaped grains.
The remaining morphologies of silver halide grains all have distinctive crystalline face arrangements. Furthermore, each face can be defined by reference to the appropriate Miller indices which, in turn, can be confirmed by a combination of visual inspection and the determination of the angle formed by the intersection of adjacent crystalline faces. This method of confirming the Miller indices of a certain crystal face may also be utilized to confirm that a given face is a {100}, {111}, or {110} crystal face.
In addition to the grain morphologies described by Maskasky, "The Seven Different Kinds of Crystal Forms of Photographic Silver Halides" Journal of Imaging Science, Vol. 30, 1986, pp. 247-255, and discussed above, which are regular morphologies--that is, they represent the seven basic homogenous crystal forms of a cubic crystal lattice--other irregular silver halide morphologies exist, these being primarily due to a combination of regular morphologies or to imperfections contained within a regular morphology.
One such irregular morphology, discussed previously, is the tabular morphology. Tabular silver halide grains have been known for years and are described in, for example, U.S. Pat. No. 4,439,520.
Cubic silver halide grains having a depression on each face and/or a hollow portion formed by joining depressions on adjacent or opposite sides of the cube are also known. U.S. Pat. No. 4,710,455 discloses such grains which can be prepared by first precipitating monodisperse cubic silver halide crystals, and then precipitating silver halide having a lower solubility than the first silver halide crystals to dissolve the first silver halide crystals.
Published Japanese Patent application 58-106532 discloses grains similar to those grains disclosed in U.S. Pat. No. 4,710,455, except that such grains are octahedral or tetradecahedral. Recesses are formed on the middle part of each grain's {111} crystal faces.
U.S. Pat. No. 5,045,443 discloses tabular grains having opposing parallel major faces that are of the {111} crystal face type. In the central region of each major face, indentations or spaces are formed.
Published Japanese Patent application 61-75337 discloses hollow silver halide grains having connecting voids having "through" holes from the surface of each grain to its inside. Such grains are formed from a "core-shell" arrangement wherein the inner "core" is susceptable to dissolution in a solvent at a faster rate than the outer "shell" region. A halide solvent such as ammonium thiocyanate is added to the grains in order to facilitate the hollowing process.
Similarly, U.S. Pat. No. 4,419,442 discloses silver halide grains having a shell of silver halide substantially surrounding a water-soluble, non-silver containing grain core. Conversion of the grains to hollow grains can be accomplished by washing the grains in an aqueous enviroment.
Published Japanese Patent application 4-56845 discloses a silver halide emulsion containing cubic or tetradecahedral grains having {100} crystal faces. 0n the center of each such face are located recesses of varying depths and sizes.
In all of the above teachings, the hollow portions or depressions in grains are not entirely encapsulated. Rather, they are, in one form or another, contiguous with the grains' surrounding environment.
U.S. Pat. No. 4,916,052 discloses hollow grains and a process for their preparation. This patent describes epitaxially growing tabular grains on a silver iodide seed crystal composed of a set of four hexagonal bipyramids that are joined at their bases to form a common tetrahedron. The tabular grains grow at the seed grain's terminations and are allowed to grow until they completely encapsulate the seed grain, thus forming substantially hollow grains.
Although it is known in the art to form hollow silver halide grains or silver halide grains having indentations on their crystal faces, it has yet been shown how to form grains having multiple internal voids. Naturally, it follows that the art has also failed to describe photographic elements containing such grains.